1. Field of the Invention
This invention relates to a magnetic sensor device that uses an inductor device.
2. Description of the Related Art
Magnetic sensors employ various kinds of sensor devices, one of which makes use of an inductor device. The inductor device is basically composed of a magnetic element serving as a core, and a copper wire wound around the magnetic element. The inductor device is typically used as one of a number of passive elements that compose an electronic circuit. A signal due to the inductance component of a magnetic element varies upon application of an external magnetic field (Hex). Magnetic sensor devices making use of this phenomenon are currently employed in many diverse fields. Basically, a copper wire wound in a coil form provides a passive element having an inductance. By inserting a magnetic element into the coil, the coil's inductance L can theoretically be increased in a way that is dependent on the permeability of the magnetic element as expressed by the following Equation (1): EQU L=.pi.a.sup.2 .mu.n.sup.2 l (1)
where a is the radius of the magnetic element; l is the length of the coil; n is the number of turns of the coil; and .mu. is the permeability of the magnetic element. By using a core (i.e., magnetic element) having a greater permeability than a hollow core, the inductance of the inductor device can be increased. It is also possible to increase the voltage that develops across the coil when the inductor device is excited by application of an alternating current. In a particular case where the inductor device is used as a magnetic sensor device, the sensitivity for detecting a magnetic field and, hence, the signal to noise ratio (S/N ratio), can be increased.
An example of the magnetic sensors that employ the inductor device described above is a flux gate (FG) sensor. Compared with other magnetic sensors, such as a Hall effect sensor, a magnetic resistance (MR) effect sensor and a giant magnetoresistance (GMR) effect sensor, the FG sensor device of current models exhibits a very high sensitivity for detecting magnetic fields. On the pages that follow, the operation of the FG sensor device is described with reference to its typical configuration, together with the current status of the magnetic sensors.
A magnetic sensor device for use in the FG sensor is generally composed of a high-permeability magnetic core made of a magnetic material and one or more coils wound around the magnetic core. Two coils are typically used, one being an exciting coil and the other being a detecting coil. Assume here that a sinusoidal alternating current having a sufficient amplitude to magnetically saturate the core is applied to the exciting coil around the core. In the absence of an external magnetic field, the BH characteristic of the core, which is symmetrical with respect to the origin, allows the detecting coil to develop a voltage waveform containing only higher harmonics of xth orders (x being an odd number). If an external magnetic field is applied, the BH characteristic of the core becomes asymmetrical with respect to the origin, and the voltage induced in the detecting coil will contain not only higher harmonics of xth orders (x being an odd number) but also those of mth orders (m being an even number). Since the amplitude of these harmonics is proportional to the change in the applied external magnetic field, the magnetic sensor is highly sensitive to detecting the change in that amplitude.
As just described above, the FG sensor in common use takes advantage of the magnetic saturation characteristic of a high-permeability core. While the inductor device is basically composed of a magnetic element serving as a core and a copper wire wound around the magnetic element, the methods of excitation and detection are by no means limited to those described above.
In another example, both ends of the coil are excited by application of an alternating current, and the phase of the voltage generated across the coil is detected, thus providing a magnetic sensor that operates on the basic principle of the flux gate sensor. In yet another example, both ends of the coil are excited by application of an alternating current such as-to produce a sufficient magnetic field to magnetically saturate the core. The amplitude change in the inductance of the voltage generated across the coil due to the applied external magnetic field is detected, thus providing a magnetic sensor of high sensitivity. In short, the magnetic sensor device used in magnetic sensors employing various methods of detection typified by the method used by the FG sensor is basically composed of a core made of a magnetic material around which a wire such as a copper wire with an insulation coating is wound. Magnetic sensors employing detection methods typified by the method used by the FG sensor are used in many quantities in diverse fields because they are highly sensitive in detecting magnetic fields.
As already mentioned, the magnetic sensor device is basically composed of a magnetic element and a wire such as a copper wire that is wound around the magnetic element. Because of this great simplicity in structure, the magnetic sensor device has the advantage of suffering from only a few drawbacks. If the magnetic element in the magnetic sensor device has a large cross-sectional area, the wire such as a copper wire is directly wound around the element. If the magnetic element has a small cross-sectional area, the wire is wound onto a bobbin-shaped frame molded from resin, and the magnetic element is inserted into the frame.
Magnetic sensors are currently used in the electronics and information fields and in various industrial machines, so they are required to be compact and yet have high sensitivity. Magnetic sensor devices are applied as the sensing head of magnetic sensors that are employed in large quantities in information fields (e.g., computers) and instrumentation which recently has seen rapid technical advances. In particular, there exists a strong need for miniaturizing the magnetic sensor device that is the heart of magnetic sensors. However, the ferritic core, which is commonly used as a highly sensitive magnetic core in the FG sensor, is a sintered ceramic body and, hence, is vulnerable to impact. In addition, it is practically impossible to manufacture sintered bodies of small cross-sectional area, and no successful attempts have been made to miniaturize the magnetic sensor device.
Even if direct winding is practiced, the wire directly wound around the magnetic element exerts a greater than negligible stress on the element. Magnetic materials generally have such a property that their shape changes when they are magnetized by application of a magnetic field; this phenomenon is called "magnetostriction." The magnitude of magnetostriction is defined as .delta.l/l (saturation magnetostriction constant) where l is the length of the magnetic material and .delta.l is the saturated value of the change in length of the magnetic material due to the applied magnetic field. The sign of this saturation magnetostriction constant is positive if the length of the magnetic material increases in the direction in which the magnetic field is applied, and it is negative if the length decreases in the same direction.
Conversely, if a stress is applied to a magnetostrictive material, its magnetic characteristics will change. The practical implication of the effect of magnetostriction is that the greater the magnetostriction constant a given magnetic material has, the greater the change that occurs in its magnetic characteristics upon application of a stress. This is problematic in that a wire wound directly around the magnetic material will exert a winding stress on the magnetic material, causing fluctuations in its magnetic characteristics.
As a further difficulty, even if direct winding causes a constant stress to be applied to the magnetic material per unit area, a large amount of force will act on the magnetic material if the material has a very small cross-sectional area. The stress of the direct winding may potentially affect the magnetic material in its entirety. For these reasons, direct winding is not currently applied to magnetic materials having a very small cross-sectional area.
The inability to reduce the cross-sectional area of the magnetic element, which is used as a magnetic core, has been a great obstacle to miniaturizing the magnetic sensor device that uses the magnetic element. More specifically, in the FG sensor device and other magnetic sensor devices of a similar type that excite the magnetic material by means of a coil wound around the material and that use the resulting change in the material's magnetic characteristics to detect an externally applied magnetic field, the ratio (l/S) of the length l of the magnetic material in the direction of its excitation with the coil to its cross-sectional area S (l/S) is critical. The value of l/S determines whether a highly sensitive magnetic sensor device can be fabricated. If the ratio of the length l of the magnetic material to its cross-sectional area S is unduly small, a great demagnetizing field Hd develops, making it impossible to achieve effective excitation of the magnetic material. Hence, in order to ensure that a magnetic element working as a core having a coil wound circumferentially therearound is effectively excited, the demagnetizing field coefficient Nd has to be reduced.
The magnitude of a demagnetizing field coefficient Hd is proportional to the strength of the magnetic poles and can be expressed by: EQU Hd=-NdI/.mu.O (2)
where .mu.O is the permeability in a vacuum and I is magnetization. According to Equation (2), the smaller the demagnetizing field coefficient Nd, the greater the effectiveness with which the magnetic material serving as a core can be excited. It should also be noted that the value of the demagnetizing field coefficient Nd varies with the shape of the magnetic material to be magnetized. Suppose here that a magnetic element having a longer-to-shorter axis ratio of 100 is magnetized in the direction of its longer axis. If the magnetic element is rod-shaped, it has a demagnetizing field coefficient Nd of 0.00036. If the magnetic element is a prolate ellipsoid of revolution (i.e., cigar-shaped), Nd is 0.00043. In the case of a flattened ellipsoid of revolution (i.e., disk-shaped), the value of Nd is 0.00776. FIG. 1 shows in experimental values the relationship between the demagnetizing field coefficient Nd and the ratio of the longer axis of a rod-shaped magnetic element to its shorter axis. Magnetic sensor devices in current commercial grades of FG sensors are generally at least 20 mm long. However, the need for smaller dimensions, particularly in length, has recently become one of the strong requirements in the field of electronic components. A maximum length that is tolerated for one magnetic sensor device is about 10 mm.
Suppose here that a wire is directly wound around a magnetic core having a length of 10 mm. If the demagnetizing field coefficient Nd is limited to not more than 1% (0.01) in order to achieve effective excitation of the magnetic core, one can see from FIG. 1 that the ratio of the longer axis of the magnetic-core to its shorter axis is about 12. Therefore, the cross-sectional area of the magnetic core must be 0.55 mm.sup.2 and smaller.
For the reasons stated above associated with the strength of the magnetic material and its magnetostrictive property, it has heretofore been necessary to use a magnetic material of small cross-sectional area in combination with a bobbin-like frame separately molded from a resin as a winding support. With this frame, a sensor device can be fabricated without considering the strength of the magnetic material and the magnitude of the magnetostrictive property it experiences. However, this frame does not satisfy the primary objective of miniaturizing magnetic sensor devices.
An object, therefore, of the present invention is to provide a compact magnetic sensor device using an inductance device.